1. Field of the Invention
This invention relates to multilayered antireflective coatings, articles having such coatings, and methods for making same.
2. Description of the Technology
It is known that a surface will reflect light and that the amount of light reflected from the surface will increase as the viewing angle increases. The reflected light from the surface of a substrate may be objectionable when an observer is viewing a transmitted image through the substrate, e.g., a cathode ray-tube (hereafter xe2x80x9cCRTxe2x80x9d), architectural glazing, a glass covering a picture, or the lens of an optical instrument or eyeglasses. The reflected light is objectionable because it obscures the transmitted image, and the transmitted image may become indiscernible if the reflected light is too intense. Viewing an image that is obscured by reflected light may cause eye strain for the observer. Further, the reflectance light may be objectionable for aesthetic reasons, as for example in the case of architectural glazing and eyeglasses.
One technique to reduce reflectance is to roughen the surface of the substrate to provide a rough anti-glare surface or to deposit an anti-glare coating over the surface of the substrate. In this manner, reflected light is scattered and appears blurred and less intense to the observer. One limitation with these approaches is that the transmitted image is also blurred due to the roughened surface or anti-glare coating.
Another technique to reduce reflection is the use of an antireflective coating. As used herein the term xe2x80x9cantireflective coatingxe2x80x9d refers to a multilayered coating that has destructive interference of light waves traveling through the coating to reduce the intensity of the reflected light. An advantage of anti-reflection coatings is the contrast of the transmitted image is improved over the image transmitted through a substrate having a rough surface or an anti-glare coating.
Certain surfaces are subject to a build up of static charges which attracts dust and/or dirt to the surface. viewing a transmitted image through a dusty and/or dirty surface blurs the transmitted image and increases the intensity of the reflected light further reducing viewing of the transmitted image. It has been found that the build up of static charge can be eliminated by grounding an anti-static coating that has a resistivity of less than about 108 ohm/square to dissipate the electric charging. One limitation of anti-static coatings is that they may increase the reflectance of light due to differences in the refractive index between the substrate and the anti-static coating.
Manufacturers of CRTs have combined the anti-glare and anti-static technologies to provide products having anti-glare, anti-static (AGAS) coated surfaces (U.S. Pat. No. 5,427,818 and 5,404,073). A drawback of products having AGAS surfaces is transmitted image degradation due to light scattering from the anti-glare portion of the coating.
The degradation of the transmitted image in anti-glare and AGAS technology may be circumvented by using an anti-reflection portion for example of the type disclosed in European Patent Application 0 533 030 A2 or replacing the anti-glare portion of the AGAS coating with an anti-reflection portion for example as discussed in Japanese Kokai Patent Application No. HEI 6[1994]-103928.
As may be appreciated by those skilled in the art, there is a need for an antireflective coating that does not have the drawbacks and limitation of the presently available coatings.
The present invention relates to multilayer coatings that reduce the percent of reflectance from a surface. In one embodiment of the invention, a multilayered antireflective coating includes at least one transparent conductive oxide layer (hereafter xe2x80x9cTCO layerxe2x80x9d) deposited over a substrate and at least one layer having an index of refraction lower than the index of refraction of the TCO layer deposited over the TCO layer.
In an embodiment of the invention, the TOC layer has a sufficient thickness to have a sheet resistivity less than about 1000 ohms/square to provide the TCO layer with anti-static and electromagnetic shielding properties. The TCO layer may be of indium tin oxide, fluorine-doped zinc oxide, zinc stannate, antimony-doped tin oxide or fluorine-doped tin oxide.
Further, the invention relates to multilayered antireflective coating having at least one layer deposited over a surface of a substrates. The at least one layer has an optical thickness approximately equal to one-third of the wavelength of the light energy whose reflection is to be suppressed (xe2x80x9cselected wavelengthxe2x80x9d), and a layer over the at least one layer having an optical thickness approximately equal to one-fifth of the above selected wavelength.
The layer farthest from the substrate (the layer having a low refractive index) preferably has a bulk refractive index less than about 1.65. The refractive index of the low refractive index layer may be homogeneous throughout its thickness or may be varied as a function of layer thickness.
The invention still further relates to an antireflective coating for substrates having a range of index of refraction at its an anti-reflection coating that reduces the reflectance of a low index (e.g., 1.4 to 1.7) surface, e.g., glass or plastic, by 75% to 90%. The antireflective coating includes a first layer having refractive index of about 1.7 to 2.2 and an optical thickness of about ⅓ a selected wavelength, and a second layer deposited over the first layer having an index of refraction that is lower than that of the first layer and an optical thickness less than about ⅓ the selected wavelength. The second layer may have a refractive index that is homogeneous or a refractive index that is graded or non-homogeneous. The first layer may be a TCO layer to provide anti-static and/or electromagnetic shielding properties.
In another embodiment of the invention an at least 4-layer anti-reflection coating to reduce the reflectance of the substrate is deposited on a low index substrate, e.g., glass or plastic, in a broad-band manner in the visible electromagnetic spectrum. A first layer deposited over a surface of the substrate has a refractive index that is higher than the index of refraction of the substrate. A second layer having a refractive index lower than the first layer is deposited over the first layer. A third layer having a thickness greater than 5 xc3x85 has a refractive index in the range of 1.7 to 2.2 is deposited over the second layer; the third layer may be electrically conductive. The fourth layer deposited over the third layer has a refractive index lower than the index of refraction of the third layer.
In yet another embodiment of the invention an anti-reflection coating deposited over a substrate includes moderately light absorbing materials as part of a multi-layer coating to improve the contrast of a transmitted image viewed through the substrate. xe2x80x9cModerately light absorbingxe2x80x9d is defined by the imaginary refractive index being in the range of 0.05-0.8. The visible light transmission through the coating can be reduced by the use of moderately absorbing materials.